What is measured
The test measures 25-hydroxyvitamin D, written as 25(OH)D, the main storage form in the blood.
It is used because it reflects long-term stores from both sources, sun exposure and intake through food or supplements. The active hormone form is not used to judge status, because it can stay normal even when stores are already low.
Results are reported in nmol/L or ng/mL depending on the laboratory, so check the units carefully when comparing an old result with a new one.
How the test is done
One blood sample. No fasting is required, and it can be combined with other panels in the same draw.
If you have already started a supplement, say what dose and for how long. Blood levels take time to rise, so a repeat test is usually done after about three months on the same dose for the result to mean anything.
How results are read
The widely used ranges split results into deficient, insufficient and sufficient, but the exact cut-offs differ between laboratories and between guideline bodies.
A few things are commonly misread:
- One low reading does not automatically call for a high dose. Dosing is decided by the doctor based on the level, body weight and other conditions.
- Higher is not better. Long-term high-dose intake without monitoring can raise blood calcium.
- An old result in ng/mL and a new one in nmol/L cannot be compared directly without converting the units.
